A swing hydraulic circuit for use in a hydraulic power shovel which has generally been used is shown in FIG. 5. A discharging flow conduit 102 of a main hydraulic pump 101 is connected to a main relief valve 103 and through a pilot directional control valve (hereafter referred to as "directional control valve") 104, which is switchable as required, to the side flow conduits 106a and 106b of a hydraulic motor 105 for turning a turning frame 121 which mounts a working machine boom 120, and a return oil conduit 108 leading to a tank 107. The side flow conduits 106a and 106b are switchably connected, as required, through the directional control valve 104 to a return oil conduit 109 leading to the tank 107.
A remote control turning valve 130 is provided with a pair of variable reducing valves 131a and 131b and an operation lever 132. A pilot hydraulic pump 140 is connected to the primary side of each of the variable reducing valves 131a and 131b, and the flow conduits 133a and 133b are connected to respective secondary sides thereof. The pilot flow conduits 133a and 133b are connected, respectively, to the pilot ports 104a and 104b, located at opposite ends of the directional control valve 104.
A relief valve 111a is provided between the flow conduit 106a and a return oil conduit 110 to the oil reservoir 107, and a relief valve 111b is provided between the flow conduit 106b and the return oil conduit 110 to the oil reservoir 107.
Check valves 112a and 112b, for preventing cavitation, are provided in parallel with relief valves 111a and 111b, respectively.
In a construction as described above, when, for example, the operation lever 132 is operated from a neutral position N to the R direction, a pilot pressure is guided from the variable reducing valve 131a to the pilot flow passage 133a and further guided to the pilot port 104a of the directional control valve 104, so that the directional control valve 104 is changed over from the neutral position N to a position L. A discharging oil of the hydraulic pump 101 flows through the directional control valve 104 into the oil conduit 106a and then into the hydraulic motor 105; at the same time the hydraulic pressure rises up to the set pressure of the relief valve 111a. The discharging oil from the hydraulic motor 105 is returned to the oil reservoir 107 through the flow conduit 106b, the directional control valve 14, and the return oil conduit 109. Thus, the hydraulic motor 105 is driven and accelerated, and the turning frame 121 is turned and accelerated.
When the operation lever 132 is reset to the neutral position N, the pilot pressure which has been guided to the pilot port of the directional control valve 104 is shut off and the directional control valve 104 is reset to the neutral position N. Then, the discharging oil from the hydraulic pump 101 is returned to the oil reservoir 107 through the return oil passage 108 and does not flow into the hydraulic motor 105. However, due to an inertial force of the turning frame 121, the hydraulic motor 105 sucks hydraulic oil from the oil reservoir 107 through the flow passage 106a, the check valve 112a and the return oil passage 110. When the pressure of this hydraulic oil is boosted to a set pressure of the relief valve 111b, the hydraulic oil is returned to the oil reservoir 107 through the flow passage 106b, the relief valve 111b, and the return oil passage 110. In this case, the hydraulic oil is returned to the oil reservoir 107 against the present pressure of the relief valve 111b to act as a resistance to the rotation of the hydraulic motor 105. Therefore, the inertial force of the turning frame 121 is absorbed and the rotation of the hydraulic motor 105 is stopped.
However, when the tilting angle .theta. of the boom 120 of the working machine installed on the turning frame 121 is small and the reach of the working machine is set at the maximum, the inertial force of the turning frame 121 greatly differs from that when the tilting angle .theta. is large and the reach of the working machine is set at the minimum. If the set pressure of the relief valve 111b is fixed, the starting torque and the stopping torque (the torque to stop rotation of the hydraulic motor 105 according to the set pressure of the relief valve 111b) of the hydraulic motor 105 are excessively large as compared with the inertial force of the turning frame 121 when the reach of the working machine is at a minimum, and therefore a shock at the time of starting and stopping is large, as shown in FIG. 6A. On the other hand, there is a problem in that, if the set pressure of the relief valve 111b is decreased in accordance with the shorter reach of the working machine 120 in order to reduce the shock, the inertial force of the turning frame 121 is excessively large when the reach of the working machine 120 is at a maximum, and starting and stopping will be delayed, as shown in FIG. 6B.